Solid state image sensors include a linear or matrix array of photosensitive elements which convert a light image incident upon the photosensitive elements into a video signal corresponding to the light signal. A typical solid state image sensor is a full frame CCD image sensor. The sensor is exposed to a light image which converts the light intensity to a charge distribution accumulated by the photosensitive array. At the end of the exposure period, the charge accumulated by the photosensitive array is serially read out from the sensor as a series of video signals having a reset level signal and a video information level signal. One problem with a solid state image sensor is that the process of reading out the video signal from the sensor introduces noise components into the output video signal, which is undesirable.
Commonly assigned U.S. Pat. No. 4,987,321, issued Jan. 22, 1991, inventor Toohey, discloses a processing circuit for an image sensor which cancels noise and distortion from an imaging system which has a solid state CCD sensor. The processing circuit includes four sample and hold circuits; two buffer circuits; a pair of source followers, which are all on a common semiconductor substrate; and further has a differential amplifier which is not on the common substrate. Although this processing circuit is useful for its intended purpose, it would be desirable to provide a noise reduction circuit which is simpler and less expensive.
The following U.S. Patents disclose various multiple sample and hold signal processing circuits which do not provide a solution to the problem of providing a simple and less expensive noise reduction circuit for a video signal produced by a solid state image sensor. U.S. Pat. No. 4,717,883, issued Jan. 5, 1988, inventor Browning; U.S. Pat. No. 4,446,486, issued May 1, 1984, inventor Itoh; U.S. Pat. No. 5,081,372, issued Jan. 14, 1992, inventor Pelgrom; and U.S. Pat. No. 4,631,518, issued Dec. 23, 1986, inventor Caspell.